Who uses Earth orientation information?

Anyone who must relate changing aspects of reference systems on
Earth to a system in space requires information on the Earth's
orientation. Users include navigators, geodesists, and those involved in
timekeeping. For navigators using celestial navigation, failure to account
for the changing UT1, in particular, could result in significant positional
errors.

Other users are those concerned with navigation in space. Precise
positions of satellites located hundreds to tens of thousands of kilometers
away in space must take into account the Earth's variable rotation. Without this
information, the positions of the satellites cannot be accurately related to points on
the Earth's surface, which is necessary for many space-based applications.
Increasing use of artificial satellites for world-wide navigation and
communication makes this area an important concern.